In many biological conversion processes such as biological leaching, fermentation and sewage treatment, it is necessary to introduce components required for growth of microorganisms into a liquid. This is particularly important when the required component is a gas such as oxygen. Numerous techniques have been developed to increase the efficiency of mixing required components in a biological conversion medium. Some of these techniques and devices are described in a review article by K. Schugerl, titled "New bioreactors for aerobic processes," International Chemical Engineering, Vol. 22, No. 4, October, 1982.
Many of these known devices are suitable for small scale or laboratory operations, but fail when scaled-up for commercial purposes. Additionally, when the medium contains solids in suspension in addition to the microorganisms rather than a simple homogenous liquid medium, the efficiency of these devices drops significantly. Therefore if a commercial scale-up is desired, for example, to recover precious metal from an ore, these prior devices are generally unsuitable for economic reasons. To be commercially feasible, large quantities of the ore must be processed which obviously requires a reactor vessel much larger than would be found in a laboratory example. The energy usage required for large scale commercial reactors using devices acceptable for small scale or laboratory work would likely be prohibitive.
Of particular interest herein, it has been found that the biological leaching of an ore allows an increased recovery of precious metal from the ore that had previously been extremely difficult if not impossible to obtain on a commercial level. In order for the biological leaching process to be effective, it is necessary to provide the microorganisms with sufficient oxygen to support the required reactions. Thus, providing the microorganism with sufficient oxygen is a critical element in biological leaching of ores and must be conducted in a manner to substantially optimize the desired reactions in order to be economically advantageous.
One device used in the biological leaching of ore is disclosed in U.S. Pat. No. 4,728,082 to Emmett, Jr. et al , March 1, 1988. The Emmett device comprises a plurality of rotating air diffusers in the bottom of a reactor vessel. The specially designed diffusers form air bubbles of a minute diameter which allow greater interface between the ore slurry and the air. The diffusers are also arranged to reduce the likelihood of clogging thereof by rotating the diffusers in the slurry. A bridge support structure is positioned over the top of the vessel for rotation thereabout. The bridge structure has a plurality of rifle tubes in cylindrical housings extending radially from a central shaft. The rifle tubes have saw-toothed floor structures which function to trap solids within the slurry as the slurry flows therethrough. While the Emmett et al. apparatus aerates a bio-leaching process, it requires specially designed components and relatively high energy pumps and is therefore relatively expensive.
Another apparatus, used for combining liquids and gases, is disclosed in U.S. Pat. No. 1,808,956 to Ketterer, June 9, 1931. The Ketterer device comprises a closed tank which holds a liquid to be treated with a gas. The liquid is drawn from the tank proximate the surface of the liquid therein by a pump. The pump forces the liquid into a looped path above the surface of the liquid in the tank and discharges the liquid through a liquid jet gas exhauster into a venturi. The effect of the liquid jets entering the venturi causes the gas, which is injected proximate the venturi, to be sucked into the venturi for mixing with the liquid. The mixed gas and liquid then flow through a pipe for reinjection into the tank. The liquid and gas mixture is forced through small openings in a pipe proximate the bottom of the tank which causes the formation of tiny bubbles and increases the interface between the liquid and the gas for further mixture thereof. The use of a venturi makes this device ineffective for use with a solids/liquid suspension such as found in an ore slurry.
Another apparatus for dispersing a gas into a liquid is disclosed in U.S. Pat. No. 2,521,215 to Haddeland et al., Sept. 5, 1950. The Haddeland device comprises a closed tank having a discharge pipe proximate the bottom thereof and an inlet pipe proximate the top thereof. The liquid in the tank is pulled through the discharge pipe by any appropriate device such as a pump. After passing through the pump device, the liquid is injected with a pressurized gas prior to entering a mixing device such as an impeller or paddle wheel. The mixed gas and liquid is then sent back into the tank through the inlet pipe near the top of the tank.
A bioreactor for mixing a gas into a fermentation medium is disclosed in the aforementioned article by K. Schugerl and is also cited by Bailey in "Biochemical Engineering Fundamentals", 2d Ed., McGraw-Hill, 1986. The reactor comprises bubble columns with stage separating trays, external tubular loops filled with static mixers and with pneumatically imposed liquid pulsation. Gas is bubbled through the medium in the reactor from the bottom to the top thereof. Additionally, a portion of the medium in an external loop is injected with gas; the gas and medium are then further agitated by the static mixer.
Other devices such as disclosed in U.S. Pat. No. 3,424,443 to Thayer, Jan. 28, 1969, and U.S. Pat. No. 3,947,359 to Laurie, Mar. 30, 1976, provide improved injection of gas into liquids by use of pipes with holes therein. The gas and the liquid are pumped into the pipe and out through the holes to further mingle with the liquid. Due to the minute size of the gas bubbles, there is increased interface between the gas and the liquid. These devices in themselves do not provide sufficient aeration and agitation for a biological conversion medium comprising an ore slurry and microorganisms.
As previously indicated above, the oxygenation of microorganisms in an ore slurry in which solids are added to a liquid is more difficult to obtain with the same degree of efficiency as with a liquid. The known methods for adding a gas to a liquid, some of which are referenced above, may obtain as high as 90% efficiency of gas absorption. However, when these same methods are used to add a gas to a solid/liquid suspension, the efficiency drops to 5%-15%.
The known devices have the disadvantage of being relatively expensive or not providing adequate means for supplying a component to microorganisms in a medium. Many of the known devices also are generally unsuitable for use with large quantities of a medium as they require a closed or sealed tank or are inappropriate when scaled-up to commercial operations. Thus, there is a need for a method and apparatus that can efficiently provide a component to a biological conversion medium in general and specifically can provide sufficient oxygen to microorganisms in an ore leaching process.